Just pasting some dialogue I came across when I entered The Solar the other day. Can't believe I missed this. I always forget about the existence of The Solar or The Rising Stones after completing the MSQ.
Regarding Unukalhai
Unukalhai: I am Unukalhai, and I serve a friend.
Unukalhai: Lest you wonder why one so young was chosen for this task, it is because I have... How shall I put this? An intimate knowledge of the primal mind.
Unukalhai: You are curious, I am sure. But you need not understand the nature of a power to use it, do you? It is much the same with the Echo, I imagine.
Regarding Primals
Unukalhai: Ifrit, Titan, Garuda, Leviathan, Ramuh, Shiva. Born of the land and returned to it.
Unukalhai: Six primals, one for every elemental aspect. So where does that leave Ravana and Bismarck?
Unukalhai: Earth and fire. Water and wind. Your latest foes are aspected to not one element but two.
Unukalhai: A rather simple but noteworthy distinction, for it allows them to feed upon aether that much more efficiently.
Unukalhai: But, like their predecessors, there is but one recourse: they must be felled, and you are the one to whom we must turn to see it done.
Unukalhai: However, unlike before, I do not think this the work of those Ascians...
Unukalhai: It has ever been said that Eorzea is a land embraced by gods. For weal or for woe, I should wonder.
Regarding Eikons
Unukalhai: Ah, yes. The beings the Allagans dubbed eikons. How much we have yet to learn.
Unukalhai: Precisely how they stand apart from primals we have yet to determine. If they were summoned in a similar manner, then who was responsible, and how have they managed to endure for millennia?
Unukalhai: We have gained some understanding of the Dark Divinity's true nature. Yet he is but one of many, and far from the most powerful.
Unukalhai: The Warring Triad, whom the Allagans imprisoned within the Centrifugal Crystal Engine of their research facility, concerns me most of all.
Unukalhai: Their infinite power is beyond comprehension. Were they released into this world, unimaginable destruction would ensue.
Unukalhai: Simply put, that cannot be allowed to happen. The very fate of this star hangs in the balance.
Anyway, according to Unukalhai, Primals and Eikons do not differ just by names, unlike what several people have said in this forums. It seems that Eikons are the ones that have existed in a summoned state for many years, making him question how have they been able to endure such a long time assuming they were summoned similarly to Primals. I guess this refers to how much aether they would have consumed if their summoning method is the same as Primals.